The quality and mechanical properties of a metal casting depend greatly on its metallurgical structure. In metals which contain nonmetallic second phases, the structure of both the initial or as-cast grain structure as well as the structure of the second phases is important because they both influence the quality and mechanical properties of the metal casting. In castings of aluminum alloys which contain silicon, or in iron alloys which contain carbon, the metallurgical structure of the nonmetallic second phase (silicon and carbon, respectively) can be altered by the addition of certain elements which affect crystallization behavior and thereby the metallurgical structure. These additions can change the nucleation and/or the growth characteristic while refining the grain size and/or the eutectic structure in a manner to improve the overall properties of the casting material. Knowing how much and when to make such additions is a difficult problem.
The level of grain refinement and eutectic modification have been evaluated traditionally by metallographic techniques requiring lengthy off-line sample preparation and examination with an optical microscope. Such traditional method involves either sectioning actual castings or sectioning a small sample of the metal which has been solidified. In either case, it is laborious to examine it metallographically. After examination and comparison to standardized photomicrographs, recommended additions can be made to the molten metal so that the desired grain structure and nonmetallic phase structure is promoted. Due to the variation of both parameters with time, the laboratory sample often differs markedly from the melt at the time the actual production casting is poured. This results in inaccurate additions that are needed for the actual production casting.
It would be helpful if some type of rapid on-line thermal analysis could accurately predict the amount of additions to be made. It is well established that major characteristics of the final cast structure are determined during solidification and, therefore, are reflected in the metal's cooling curve, a cooling curve being a plot of the variation of temperature with the lapse of time (see U.S. Pat. Nos. 3,478,808; 3,358,743; 3,991,808). However, it has been difficult to quantify the relationships between the metallurgical structure and relevant parts of the cooling curve, especially within the time constraints of the production environment. In the past few years an attempt has been made to do just that. In U.S. Pat. No. 4,333,512 a method was developed whereby the difference between the lowest and highest temperatures (temperature difference .DELTA.T) within a specfic phase transition region of the cooling curve is measured and related to previous trials which indicate what metallurgical structures will be obtained with a specific temperature difference at such curve portion. The beginning phase transition temperature and ending phase transition temperature are compared to render the sensed temperature difference, .DELTA.T. No attempt was made to relate such measured .DELTA.T value to time. Similarly, in a French publication (see sales brochure of Societe De Vente De L'Aluminum Pechiney, 1982), the same type of measurement of the temperature difference at a specific phase transition region of the cooling curve was measured and related to the metallurgical structure for the same type of temperature difference (.DELTA.T). With the French method, a thermocouple is embedded in an interchangeable crucible used to make solidification samples of a melt. The sensed temperature difference (.DELTA.T) at selected portions of the cooling curve is then compared against one standard temperature difference (.DELTA.T) representing the desired metallurgical structure.
The above mentioned methods are quite adequate if the transition temperatures and, therefore, the resultant temperature difference (.DELTA.T) of the solidifying alloy are consistent, accurately measurable, and affected only by the structure. However, the measured transition temperatures and, therefore, the temperature difference (.DELTA.T) can also be affected by other factors, notably the chemistry of the alloy. Thus, a misleading prediction of the structure may result if, for example, there is a fluctuation in the alloy chemistry among samples or if the transition temperatures cannot be measured accurately due to some other reasons. As an example, what may happen is that the measured difference (.DELTA.T) between two samples will not represent different metallurgical structures but actually be due to only a fluctuation in alloy chemistry with generally the same metallurgical structures.
It would be highly desirable to develop a rapid on-line melt monitoring method which can tolerate some inconsistencies in phase transition temperatures due to fluctuation in alloy chemistry or difficulties in obtaining accurate transition temperatures.